1. Field of the Invention
The present invention relates to the removal of manganese ions from acidic sulfate solutions, and more specifically to the removal and control of manganese in neutral leach or weak acid leach solutions and circuits. Using the method of the present invention, manganese can be precipitated from the leach solution containing valuable non-ferrous metals, such as copper, nickel, cobalt, zinc, or combinations of these, without causing the metal(s) of interest to precipitate from the solution. The present invention also relates to an apparatus for treating an acidic sulfate solution, and preferably a zinc leach solution, to selectively remove manganese without precipitating zinc from the solution.
2. Description of The Related Art
The great majority of zinc is produced via hydrometallurgical processing of zinc sulfide concentrates. Typically, such concentrates are produced via flotation, and are subjected to roasting, followed by leaching in sulfuric acid solution, purification of the solution, and recovery of metal via electrolysis (electrowinning). As an alternative to roasting, direct leaching of the concentrate either in autoclaves (i.e. pressure leaching) or under atmospheric pressure may be practiced. In all of these processing methods, any manganese in the zinc feed tends to dissolve and build-up in the solution. Hence, zinc plant operators are particularly careful to select concentrates for treatment that are low in manganese content.
In addition to the feed itself, another source of manganese that may contribute to its build-up in zinc solution is manganese-based oxidants (such as pyrolusite or permanganate) that are used to facilitate the oxidation of ferrous iron to ferric iron, and hence its subsequent removal via precipitation (as ferric hydroxide-ferrihydrite in the neutral leach stage or jarosite, goethite or hematite in the hot acid leach stage(s)). Manganese enters the solution in its soluble divalent state and accumulates, causing problems with the operation of the plant, especially in the tankhouse. Some manganese (II) is desirable in the electrolyte (approximately 2 to 5 g/L), as it results in the deposition of a protective MnO2 coating on the lead anodes that otherwise undergo rapid corrosion.
Bleed off of electrolyte, in addition to manganese deposition on the anodes, has been the conventional means of controlling manganese in zinc process solution when the feeds processed are very low in manganese content. However, such an approach is not adequate if concentrates rich in manganese are to be processed. For this case, a chemical method that will remove manganese selectively while leaving zinc in solution needs to be devised and implemented, so as to prevent the unacceptable excessive build-up of manganese.
In the past, the chemical removal of manganese from acidic sulfate solutions, particularly those containing zinc, was accomplished via the use of strong oxidants such as ozone, Caro""s Acid, or potassium permanganate, although these methods are not well-suited for industrial use. Of these, the use of ozone to cause the oxidative precipitation and removal of manganese from zinc-containing sulphate solutions in the form of manganese dioxide and the separation of it from solution by filtration has been described in U.S. Pat. No. 4,290,866 and its companion patent No. 4,379,037. According to this patent, spent electrolyte (and not the leach solution), i.e., the solution exiting the tankhouse containing approximately 50 g/l Zn(II) and 180 g/L H2SO4 is treated with ozone to remove manganese via oxidative precipitation. Although this method is technically feasible, it does, however, suffer from the high cost of the oxidant used. The same drawback is associated with the use of Caro""s acid or potassium permanganate.
Use of SO2xe2x80x94O2 gaseous mixtures has been reported in literature for the oxidation of a number of substances, such as cyanide, ferrous iron, nickel (See E. A. Devuyst et al., Hydrometallurgy: Research, Development and Plant Practice, published by TMS, Warrendale, Pa. (1983), pp. 391-403), and arsenic (See Q. Wang et al., Waste Processing and Recycling III, published by CIM, Montreal, QC (1998), pp375-387), but not for the selective oxidation of manganese from acidic sulfate solution such as a zinc leach solution, or a solution containing other non-ferrous metals such as copper, nickel, and cobalt, and complex concentrates containing them.
U.S. Pat. No. 4,029,498 describes the use of SO2/O2 (Air) to remove manganese from solution, but the solution used in that application was alkaline in nature, and was produced by the ammoniacal leaching of manganese nodules. The solutions treated using this process contained chloride salts in addition to ammonia (this is a base that makes the solution alkaline), and therefore they are distinctly different in their chemical make-up from the acidic sulfate solutions containing non-ferrous metals such as zinc, copper, nickel, and cobalt, to which the present invention applies.
The process for making manganese described in U.S. Pat. No. 5,932,086 is also clearly distinct from the process of the present invention, as it involves treating a source of manganomanganic oxide in a leach solution in the presence of a reducing agent to convert the manganomanganic oxide to manganese sulfate. The source of manganese is leached in a sulfuric acid solution using SO2 as a reducing agent. The reducing agent causes manganese (II) oxide to form, which is further converted to manganese sulfate solution from which ultimately manganese is recovered by electrolysis after a number of manipulations.
U.S. Pat. No. 2,779,659 describes a method of recovering manganese from ore by leaching with nitric acid, followed by an oxidizing step to form MnO2. Air or O2 may be used as the oxidant. The method of treating water set forth in U.S. Pat. No. 3,349,031 is also relevant to the present invention in that it utilizes compounds that yield bisulfite ions to remove manganese. The method described in this patent is directed toward use in treating water containing manganese in very small quantities, and is not suitable for use in treating an industrial leach solution.
Accordingly, there is a need for a method and apparatus for selectively removing manganese from an acidic sulfate solution containing zinc, such as a zinc leach solution. Such a method and apparatus will allow for the economic treatment of zinc concentrate feedstocks rich in manganese that are not otherwise treatable by the processes of the prior art. An additional advantage of this new method is that the manganese removal process has the potential of removing other deleterious impurities via co-precipitation, thus making the downstream purification operation even easier. This method and apparatus also provide greater ease of application.
The present invention provides a new process and system for selectively removing manganese from acidic sulfate solutions containing valuable, non-ferrous metals such as zinc, copper, nickel, and cobalt, allowing the effective control of manganese in hydrometallurgical plants. This novel process makes use of common chemical reagents (such as O2 and sulfite salts or gaseous SO2) that are abundantly and inexpensively available in such plants.